Process for initiating explosive and charge thereof

ABSTRACT

Improvement in the explosion bonding of metals which comprises initiating the layer of explosive used therefor interiorly throughout its thickness in such a manner that the metal parts to be bonded are driven together under bonding conditions in the area of initiation until the attainment of steady-state collision conditions, with little or no damage to the metals; and an explosive primer particularly useful for effecting such improvement.

United States Patent Oswald R. Bergmann Cherry Hill Township, N.J.;

Joseph Buchwald, Media, Pa.; George R. Cowan, Woodburg, NJ.

Nov. 15, 1968 Division of Ser. No. 649,887, June 29, 1967, Patent No.3,439,408.

Jan. 12, 1971 15.1. du Pont de Nemours and Company, Wilmington, Del.

a corporation of Delaware [72] Inventors [21] Appl. No. [22] Filed [45]Patented [73] Assignee [54] PROCESS FOR INITIATING EXPLOSIVE AND CHARGETHEREOF 2 Claims, 3 Drawing Figs.

[52] 11.8., C1 102/24 [51] Int. Cl F42b 1/00 [50] Field ofSearch.::.102/22 24 [56] References Cited UNITED STATES PATENTS 2,698,5751/1955 Poulter 102/24 2,757,611 8/1956 Church et al. 102/24 2,892,4076/1959 MacLeod 102/24 3,082,689 3/1963 Griffith et al. 102/24 3,280,74310/1966 Reuther 102/24, 3,439,611 4/1969 Holtzman 102/24 PrimaryExaminerVerlin R. Pendegrass Attorney-John F. Schmutz ABSTRACT:lmprovement in the explosion bonding of metals which comprisesinitiating the layer of explosive used therefor interiorly throughoutits thickness in such a manner that the metal parts to be bonded aredriven together under bonding conditions in the area of initiation untilthe attainment of steady-state collision conditions, with little or nodamage to the metals; and an explosive primer particularly useful foreffecting such improvement.

PATENTED JANI 21911 FIGJ INVENTORS 05MB R. BERGNANN JOSEPH BUCHWALDGEORGERCOWAN PROCESS FOR INITIATING EXPLOSIVE AND CHARGE 'rnannor CROSSREFERENCE TO PRIOR APPLICATION This is a division of application Ser.No. 649,887, filed June 29, 1967 and now Pat. No. 3,439,408.

BACKGROUND OF THE INVENTION The past few years have witnessed thedevelopment and commercial acceptance of metallurgically bonded cladproducts made by explosion bondingsTechniques for explosion bonding aredescribed in US. Pats. 3,137,937 and 3,264,731, and in copending,coassigned US. Pat. Application Ser. No. 503,261, now Pat.Nor-"3,397,444 Briefly, the procedure involves propelling metal layerstogether with an explosive so as to" cause them to collide progressivelyat a velocity which is below 120 percent of thesonic velocity ofprogressive collision. When the metal layers are initially substantiallyparallel, the collision velocity equals the detonation velocity of. theexplosive. When there is an initial angle between metal layers,thecollision velocity is somewhat lower than the detonation velocity,larger initial angles giving lower collision velocities with agiven'explosive, The preferred minimum angle to be produced between themetal layers on impact during the bonding'process varies from one metalsystem to another, being about 4" for some, andhigher for othersflngeneral, all other thingsbeing constant, the impact angleproduceddecreases with metal density, increases with explosive loading, andincreases with initialstandoff or angle between layers to'a maximum,thereafter decreasing with larger standoffs. Stated differently, impactangle increases with metal layer velocity at constant'collision velocityand passes'through a maximum with increasing standoff. At a givenstandoff, the metal layer velocity is increased by increasing theexplosive loading, the minimum metal layer velocity usually requiredbeing on the order of 150-400 metersper second. I a g I l The explosionbonding process described above produces a continuous metallurgicalbonding of'the metal layers. It is often difficult, however, to achievestrong bonding in the small area of the interface which is'direc'tlybeneath the location of the initiator for the explosive layer. Becauseof the special requirements of the bonding system, conventional pointinitiators, e.g., blasting caps, generally are not used. Such initiatorsoften are inadequate to assure a rapid and reliable initiationespecially of the less sensitive low-detonation-velocity claddingexplosives. Furthermore, the impulse which a small point charge at thesurface of the explosive layer imparts to the metal layer adjacent tothe explosivelayer is lower in the vicinity of the initiator than infartherfremoved areas owing to the divergence of the detonation front,this deficiency in im-v pulse in the vicinity of the initiator beingaccentuated by the low detonation-velocity and energy output of thecladding explosive in the diverging region. The low impulse tends tocause the metal layers to collide at too small an impact angle and toohigh a collision velocity for good bonding to occur in the initiatorregion. Larger initiators located inside. the explosive 2 layer adjacentto the explosive. For example, one method, that described in U.S. Pat.No. 3,258,841, uses a metal extension piece on the metal layer onwhichthe explosive is located, the explosive extending over the extensionpiece as well and initiated at the edge over the extension piece. Inthis method the extension piece is expended. Anothertechnique, describedin U.S. Pat. No. 3,140,539, is to provide a convex projection in thesurface of the cladder layer, i.e., the metal layer adjacent to theexplosive, and to initiate the explosive at a location in line with theprojection. The extension piece method obviously cannot be used when theadded expense of the consumed metal cannot be tolerated. The convexprojection method generally is troublesome to manage on a productionscale, particularly with thick cladder layers. Thus, in the interests ofgreater economy and more general applicability it would be highlydesirable to have an improved method for reducing the poorly bonded areain the vicinity of the initiator in the explosion metal cladding processwhich does not require the use of additional metals or additional stepsin the preparative stage of the cladding operation.

SUMMARY OF THE INVENTION This invention provides an improved primeruseful ina method for initiating the explosivein explosion metal bondingwhich results in a considerable reduction in the area of poor bondingusually found in the vicinity of the initiator and which, in itspreferred embodiment, provides substantially complete bonding in theinterfacial area between metal layers in the vicinity of the initiatorwith little ,or no damage or distortion in the parts being bonded. Thus,this invention provides a primer used for effecting an improvement inthe process for metal bonding by driving the metal parts to be bondedtogether progressively with a layer of explosive adjacent at least oneof the parts, which improvement comprises embedding an initiatingcharge, i.e., an explosive primer, within the explosive layer andinitiating the explosive layer interiorly throughout its thicknessprogressively from the surface thereof remote from the metal part to bedriven toward the surface thereof adjacent said part, the explosivelayer being in itiated closest to the axis-of initiation in the portionof the layer adjacent the metal, thereby driving the parts togetherunder bonding conditions in the area of initiation, i.e., from theregion under the primer until steady-state collision conditions areachieved. Preferably, the initiation of the explosive layer adjacent themetal immediately'precedes the impulse applied to the metal by theprimer along the axis of initiation so of the primer is higher than thatof the explosive layer, per unit area of the metal part adjacent to theexplosive layer and primer. i i v V i 4 Preferred primers used forefiecting the improvement of this invention byinsertion and actuationinthe explosive layer comprise an explosive charge having a smallercross-sectional layer afford improved bonding, but nevertheless arefound to produce a certain small poorly bonded area due to the fact thatthe conditions required for bonding, i.e., collision velocity, impactangle, and metal layer velocity, fail to be maintained constantly fromthe. moment of initial impact. Measures which have been suggestedin..the past for avoiding the production of a poorly bonded area beneaththe initiation point, have approached the problem by modifying the metalarea at one end than elsewhere along its longitudinal axis, detonationofthe charge in an axial direction toward said small end'along theperipheral portion thereof preceding that along the axis.

Reference herein to-a portion of. an explosive layer or primer beingadjacent? the metal layer means that portion closest to the metal, itbeingcontemplated that the explosive and printer can either be in directcontact therewith or separated therefrom by buffer orprotectivematerial, e.g., of plastic or rubber sheeting or foam. Layer is usedherein to refer to a body of explosive or metal having substantiallygreater dimensions in two directions than in the third or thickness"direction and is intended to include not only planar bodies, e.g., thelayers of explosive used in bonding flat sheets and plates, but alsocurvilinear, e.g., tubular, bodies. Steady-state collision conditionsrefer .to the conditions resulting when dynamic equilibrium-betweencolliding metal parts is established as the detonation progressesthrough the explosive layer without influence from the primer. Thecollision conditions produced in the area of initiation may besubstantially the same as the steady-state conditions so thatsteady-state conditions may prevail during the entire collision process.On the other hand, while the collision conditions produced in theinitiation area may differ from the steadystate conditions, the initialimpact conditions and those prevailing during the transition to thesteady-state are, interruption, those required for bonding as describedabove.

The area or place of initiation refers to the location in the explosivelayer where initiation of said layer takes place by detonation of theprimer embedded therein, and also to the corresponding location in themetal layers being bonded while under the influence of the detonation ofthe primer. In the present method, the explosive layer is initiatedinteriorly throughout its thickness; Thus, it is initiated at an innersurface, i.e., the surface of initiation, which conforms to theconfiguration of the outer surface of the primer embedded therein. Axisof initiation" refers to a line perpendicular to the layer of explosiveand passing substantially through the center of the cavity bounded bythe surface of initiation, i.e., through the center of the primer usedto initiate the layer. Longitudinal axis of the primer refers to theaxis thereof which coincides with the axis of initiation when the primeris inserted into the explosive layer and in which direction thedetonation progresses; in most cases, it is the longest axis of theprimer, but where the primer is to be used with thin explosive layers orwhere a flat disc of explosive is positioned at the initiation endthereof, the longitudinal axis need not necessarily be the longestdimension. Peripheral portion of the primer refers to that portiondisplaced from the longitudinal axis thereof, usually the outer portionthereof exclusive of the end at which it is actuated and the endopposite thereto.

DESCRIPTION OF THE DRAWINGS In the accompanying drawings whichillustrate specific embodiments of this invention and wherein likenumerals are used to denote like elementsf FIG. 1 is a cross-sectionalview of a primer of this invention;

FIG. 2 is a cross-sectional view of a portion of an explosion claddingassembly employing a primer in accordance with this invention; and

FIG. 3 is a schematic cross-sectional view of a portion of the assemblyshown in FIG. 2 after detonation of the primer.

DETAILED DESCRIPTION OF THE INVENTION In its broad aspects, the processis characterized by two features. First, each explosive layer employedis initiated at an inner surface throughout its thickness, rather thanby a primer or initiating device at the outer surface of the explosivelayer followed by propagation of a detonation through the layer. Thisallows greater control of detonation as it begins at the place ofinitiation, and the primer gives an increase in impulse to counteractthe loss from divergence of the detonation front as described above.Second, the initiation of the explosive layer is closest to the axis ofinitiation in the portion of the explosive layer adjacent the metal.This means that the surface of initiation of the layer, and thereforealso the outer surface of the primer which conforms thereto, is closestto the axis of initiation at the portion thereof adjacent the metal,i.e., that the cross-sectional area of the cavity bounded by the surfaceof initiation and of the primer is smaller at the portion adjacent themetal. As a result, the initial action of the primer is concentrated inthe center so that the impact angle is large enough and the collisionvelocity low enough near the axis to give good bonding. This manner ofinitiation also minimizes damage to the metal yet permits use of primersof sufficient size to assure initiation of the explosive layer with nearsteadystate velocity. Another very important advantage of the second ofthe above features is that it permits a rapid enough spread of thedetonation through the explosive layer to give a smooth transition tosteady-state conditions while bonding conditions are maintained.

The use of a special initiating explosive charge or primer in explosioncladding is often advisable to assure a rapid and reliable initiationespecially of the less sensitive of the lowdetonation-velocity claddingexplosives. For this reason, the primer should usually itself be moresensitive to initiation than the cladding explosive, and, usually atleast in the portions of the charge adjacent to the initiating devicetherefor. cg. blasting cap, the primer should have a higher detonationvelocity than the cladding explosive. At the same time, to ensure amaximum degree of bonding in the intertacial area between metal layersin the vicinity of the place of initiation, the metal layers adjacentthereto should be propelled in a manner such that the conditionsrequired for optimum bonding, i.e., the impact angle and collisionvelocity, are achieved from substantially the instant of impact andmaintained thereafter. These conditions should be met, of course. by aprimer which does not cause significant damage to the metal layeradjacent thereto. These requirements are met by primers used inaccordance with the present invention. I

In processes employing primers having a constant cross-sectional area,the pressure drops abruptly from a maximum under the primer to a valuesimilar to the steady-state detonation pressure of the main charge nearthe periphery of the primer. If the collision angle near the primer axisis quite large and the distance from the axis to its periphery is small,the adjoining low-detonation-velocity cladding explosive may not impartsufficient impulse to the adjoining portion of the metal layer to driveit at sufficient velocity to give a large enough steady-state collisionangle and the correct collision velocity for good bonding to occur.Increasing the distance from the axis of the primer to its periphery inan attempt to overcome this problem cannot be undertaken at will sincegross impact of the metal layers may result, and the risk of damagingthe metal layer becomes high with larger primers adjacent to the layer.In accordance with the present invention, the problem is solved withoutsuch risk owing to the flexibility which can be achieved in theimpulse/distance profile from the initiation axis. In the presentinvention, the decreasing load in primer charge explosive away from theinitiation axis results in a gradual change in the impulse imparted tothe cladder or driver metal from an axial impulse, usually a maximum, tothe impulse achieved with the cladding explosive alone. In theintermediate impulse region, a sufficiently high metal layer velocitycan be produced to give the required steady-state collision angle andcollision velocity. At the same time, the amount of explosive adjacentto the metal layer can be kept to a desired minimum. The gradual impulsechange achieved with the present invention also is advantageous in thatmuch less strain is produced in the metal layer than occurs with anabrupt impulse change. An additional advantage offered by the presentinvention, inherent in the use of a smaller area of primer adjacent tothe metal layer than would normally be required to achieve properbonding, is that the severity of the pressure pulse reaching the surfaceof the metal layer at which bonding is to take place is considerablyless than that of a pressure pulse produced from a larger-area charge.This also subjects the metal layer to less strain.

The primer or initiating explosive charge used to effect the process canbe tapered from the large-area end. The tapering of the charge can beginat some intermediate location on its periphery and extend to thesmaller-area end, e.g., the charge configuration can be that of a coneortruncated cone with its larger base abutting an end, or portion of anend, of a solid cylinder or disc as in FIG. I. The peripheralconfiguration of the charge, and therefore the configuration of theinner sur-.

face of initiation of the cladding explosive layer, are not critical,and the shape of the cross section normal to the axis of initiation canbe as desired, e.g., a circle, oval, rectangle, triangle, or otherpolygon. The choice of a specific charge configuration generally willdepend chiefly on how easy the body is to make, how durable it is, etc.

Primers of this invention will now be illustrated by reference to thedrawings. In FIG. 1, a primer has a frustoconical lower end portion lahaving a cone anglea and its larger base abutting the end of a solidcylindrical upper portion 1b, the cross-sectional area of the end of thecharge to be positioned adjacent the metal to be bonded being smallerthan that of the opposite end of the charge. The charge comprisesself-supporting detonating explosive 2, e.g., a cast or extruded plasticexplosive. surrounded, at its cylindrical portion, by a thin sheath of adifferent detonating explosive 3, which also covers the larger-area endsurface of the charge in the fonn of a thin disc 4. Electric blastingcap 5 is affixed to the center of disc 4 on the longitudinal axis of theprimer, indicated by the dotted line. This dual-explosive charge isuseful in cases in which the explosive used in the body of the primer isinsufficiently sensitive to initiation by a blasting cap, or does nothave a sufficiently high detonation velocity to initiate the claddingexplosive rapidly and reliably. In such cases, the sheath and disc canbe made of an explosive having a higher detonation velocity thanexplosive 2. 1

In FIG. 2, a primer 1, in thiscase a single-explosive charge having thesame configuration as the charge depicted in FIG. 1, is shown with itssmaller-area end surface resting on the surface of metal layer 7, e.g.,a plate, which is to be explosionbonded to a second metal layer.Explosive charge 1 has a higher detonation velocity and higher loadingthan the cladding explosive 8 surrounding it. 9- is aframe, e.g., onemade of wood, which is used to hold powdered explosive 8. When blastingcap 5 is actuated, the explosive in the primer is initiated, thedetonation travelling axially through primer 1 to propel metal layer 7,as well as in other directions to initiate explosive 8 at its interfacewith the primer. Because the loading of higher-velocity explosive onmetal layer 7 decreases from a maximum at the axis of initiation, thelongitudinal axis of the primer, metal layer 7 is subjected to aconcentrated initial action in the center, causing the metal to deform,as shown in FIG. 3. The gradual change in-irnpulse to the lowersteadystate impulse producedtby the charge'8 alone in adjacent regionspermits metal layer 7 to be propelled at sufficient velocity near theaxial region to set up a collision angle, B (FIG. 3), and a collisionvelocity which are in the range required for bonding at the axis andadjacent regions without causing significant damage to metal layer 7.

- The specific dimensions of the charge vary depending on such factorsas the properties of the cladding explosive and primer explosive, themass of V the metal layer to be accelerated, the yield strengths of themetals in the system, the steady-state impact angle and'velocitydesired, the configuration of the charge, etc. To achieve a desiredcollision geometry in a given system, larger charges will be requiredwith weaker (lower weight strength) primer explosives, larger drivenmetal masses, and higher yield strength metals. Larger charges arerequired to produce larger impact angles. The length of the primercharge, i.e., the distance between its larger.- and smaller-area ends,in general is substantially equal to the thickness of the claddingexplosive layer which it is to initiate. Generally, the area of the endof the charge adjacent the metal layer is as small as possible to givetherequired acceleration to the metal layer without causing'damage. Thisend of the charge for ease of use, is a surface. As a practical matter,one can first select a charge configuration, fixing the length of 'thecharge from the cladding explosive layer thickness, and select adesirable size for the smaller -area end surface. As a rule, a0.250-inch-diameter or an area of about 0.250square inch is adequate,but larger areas can be emmetal layer velocity beyond the metal areaadjacent the smaller-area end of the primer, and a collision velocity inthe range required for bonding. In general, the cross-sectional area ofthe larger portion of the primer'is at least about twice, and up toabout four times, the area of the smaller portion thereof. The ratios ofthese areas may be higher.

The variation in cross-sectional area is governed by the particularcharge configuration. In the case of conical and frustoconical charges,fixing the length and end surface areas of the charge determines therate of loading change from the periphery to the axis (i.e., determinesthe cone angle used). In other cases, the large area can be. maintainedfor a certain length of the charge, with a gradual reduction beginningthereafter, as in FIG. I. The cone angle in these cases depends on wherethe tapering begins. Preferably, the large area is maintained for atleast about 50 percent of the charge length,

and the cone angle a is less than about 85. The large-area portionshould not extend farther than about 90 percent of the charge length,and the cone anglea should be at least about 20.

The primers of this invention can be made of a single explo-, sivecomposition or two or more explosive compositions. When a singleexplosive is used, it must meet the requirements specified above, i.e.,be more readily initiated than the cladding explosive and at least aportion thereof have a higher detonation velocity than the claddingexplosive. Also, it should be provided in a higherexplosive loading thanthe cladding explosive, per unit area of the metal layer. Generally,

explosives which detonate at a moderately high velocity, e.g., in therange of about from 3000 to 7000 meters per secondl and primer explosiveto cladding explosivedetonation velocity 7 ratios of 1.5/1 to 3/1, arepreferred.The explosive can be selfployed, especially with thicker metallayers. Generally, the

longest cross-sectional dimension, i.e., dimension perpendicular to theinitiation axis, of the smaller end surface is at least about equal to,and less than about three times, the-thickness of the metallayeradjacent thereto, the larger primers being supporting, e.g., a castor plastic explosive, or a powdered explosivemaintained in a weakcontainer, e.g., cardboard. Typical of the explosives which can be usedare PETN, TNT, H MX and other organic nitrates, nitramines and nitrocompounds, inorganic azides, and mixtures of the foregoing alone or withother materials as in cast pentolite and /20 amatol.

There are some explosive compositions, e.g., cast trinitrotoluene, whichare'particularly desirable for use in the present primer in that theydetonate 'at a desired moderately high velocity and are capable ofpropelling the metal layer properly, but which are'in themselvesdifficult to initiate. Such a composition can be used as the majorconstituent of the primer with a thin layer or sheath of a cap-sensitivehighervelocity detonating explosive surrounding the composition and athin disc of the more-sensitive explosive at the initiation end of theprimer. This embodiment is shown in FIG. 1. Usually the thickness of thecap-sensitive'lay'er is about'0.02 to 0.2' inch. Preferably, the ratioof the detonation velocity of the sheath explosive to that of the majorconstituent of the primer is 1.2/1 to 3/1. When the sheath explosive hasa detonation velocity of about 6000 meters per second or higher, thesheath should extend from the large-area end of the primer to a-locationintermediate the ends of the primer extending for more than 50 percentof the. length of the charge periphery, preferably to about 75 percent,and preferably no more than about percent of the length. Extension ofthe layer of capsensitive higher-velocity detonating explosive tothe-end of the charge which will be placed adjacent to the metal layeris not preferred because stronger shock waves will thereby be producedin the metal, with possible damage thereto. Whena moderately highdetonation velocity explosive is used in the sheath, e.g., onedetonating at about 40005000 meters per second, the sheath can extendthe full length of the primer periphery. I a

The use of a higher-detonatiomvelocity explosive layer around the mainprimer explosive also is-beneficial in producing the desired angle uponimpact of the metal layers beneath the initiator. Because the detonationin the outer layer of the primer travels ahead of the detonation in thecore, the detonation front impacts against the metal layer as a ring orannulus.

This results in a slight inward motion of themetal layer toward thecenter and subsequent impact from the detonation of the central portionof the charge drives the central portion of the metal layer aheadcausing a conical deformation and angular impact. The particularexplosive composition used to form the outer layer is not criticalprovided it can initiate the lowerdetonation-velocity cladding explosiveand the core explosive used in the primer. Although cast or powderedexplosives can be used, self-supporting sheet explosives are preferredsince they can be applied easily by wrapping around the core. When adual-explosive system is used in the primer, the core explosive can be alow-velocity cladding explosive. Thus, the detonation velocity of themajor constituent of the dual-explosive primer can be equal to thedetonation velocity of the cladding explosive, although it preferablyexceeds the cladding explosive velocity by 1.5 to 3 times.

The primer has an initiating device, e.g., a blasting cap, usuallypositioned substantially at the center of the larger-area end ininitiating relationship with a peripheral layer of a more readilyinitiated explosive. Alternately, a line wave generator can be used toinitiate the periphery of the primer at its larger end.

As has been stated before, the choice of a specific primer configurationdepends chiefly on ease of production and durability. On this basis, thedesign shown in FIG. 2 is a preferred one. To provide added control onthe production of. the desired angular impact beneath the charge, andespecially if impact at a single point is desired, the primer shown inFIG. 1 is especially preferred, optionally with the use of an inertinsert beneath disc 4 to cause the peripheral layer 3 tobe initiatedbefore the axial region of explosive 2. Thus the primer of FIG. 1 yieldsmaximum bonding and minimum deformation and spalling of the driven metallayer.

The primer is positioned in the layer of cladding explosive with itslongitudinal axis normal to the'explosive layer and its smaller-area endresting substantially on the surface of the metal layer adjacent theexplosive layer, as shown in FIG. 2. While in some cases the end of thecharge will be in contact with the metal layer, a thin layer of asurface-protective material such as plastic film or tape may be used, ifdesired. The primer can be placed at any point on the surface of themetal layer, e.g. at an edge, corner, or more centrally located point.Several primers also can be placed along one edge of a large plate. Inaddition, as shown in FIG. 2, one cladder and one backer layer can bebonded or, alternately, several cladder layers can be bonded with oneexplosive layer, or, as shown in the examples, several explosive layerscan be used simultaneously. An important advantage of the presentinvention is that, since complete bonding at the initiation site ispossible, the place of initiation need not be restricted to an edge aswould be the case if a faulty bonding area occurred in the vicinity ofthe initiator and the area'had to be removed.

The collision parameters desired to be achieved upon detonation of theprimer and the initiation of the adjacent cladding explosive vary fromsystem to system and are described more fully in the aforementionedpatents and patent application, the disclosures of which areincorporated herein by reference. As a rule, a minimum impact angle ofabout 4 is desired. In the case of silver alloys, such as is describedin the Example 1, an impact angle of about 6.5-7.0 is desirable. Thisimpact angle is the steady-state impact angle which is establishedbeyond the axial region of the initiating charge in a typical claddingarrangement. The axial impact angle can be slightly higher. Usually theimpact angle varies from about 4 to 25, and preferably 5 to Impactangles and collision velocities can be measured from framing camerasequences.

As previously indicated, the particular primer configuration variesdepending on the particular cladding system used. In general for anyparticular system, a primer having a length about the depth of theexplosive layer, a small end with a diameter about I to 3 times thecladder thickness and a large portion having a diameter about 2 to 4times the small end is selected. Steady-state collision and collisionunder the place of initiation are compared by framing camera sequences,and,

if the impact angle at the place of initiation is too large or too smallwhile the collision velocity is in the proper range. the mass or powerof the, primer explosive can be reduced or increased, respectively. If,on the other hand, the collision velocity is too low, the size of thelarger-area portion of the primer can be increased. If there isexcessive deformation, primer mass can be proportionately decreased or aless powerful explosive used. By using a higher velocity explosiveadjacent the main charge in the peripheral portion of the primer or byperipheral initiation, the angle of impact is sharpened at the firstpoint of impact at the axis of initiation; hence, a poorly bonded areais substantially eliminated.

The following examples serve to illustrate specific embodiments of thepresent invention. However, they will be understood to be illustrativeonly and not as limiting the invention in any manner.

EXAMPLE 1 A three-layered clad is made as follows:

A 12 inch X 10 inch, 0.25-inch-thick /20 silver/copper alloy cladderplate is positioned on each side ofa l 2 inch X 10 inch, 1.25-inch-thick80/20 copper/silver alloy backer plate with the plate surfacessubstantially parallel to each other and facing surfaces at a standoffof 0.125 inch. A primer such as that depicted in FIG. 1 is placed on theoutside surface of each cladder plate at the center of one of thel0-inch sides of the rectangular surface. The positions of the primerson the cladder plates are corresponding positions. The configuration ofeach primer is that of a truncated circular cone having a0.5-inch-diameter smaller base, a height of 0.5 inch, and alinch-diameter larger base abutting the end of a l-inch-diameter,1.5-inch-long circular cylinder. Explosive 2 is cast trinitrotoluenedetonating at a velocity of 5500 meters per second. A 0.05-inch-thicklayer of a sheet explosive comprised of PETN in an organic rubber and athermoplastic terpene hydrocarbon resin binder (US. Pat. No. 2,999,743)and detonating at a velocity of 6900 meters per second is wrapped aroundthe cast TNT for the 1.5-inch-long cylindrical portion of the charge. A0.05-inch-thick disc of the same sheet explosive is placed on thelarge-diameter end of the charge, and a No. 6 electric blasting capaffixed to the center of the disc.

The primer is placed on the plate with the small base of the conecontacting the plate and abutting the plate edge. A wooden frame isplaced around the outer surface of each cladder plate, and a granularexplosive composition comprised of 16 percent trinitrotoluene, 74percent sodium nitrate, and 10 percent sodium chloride is packed intothe frame so as to form a 2-inch-thick layer detonating at a velocity of2l002400 meters per second. The granular explosive surrounds the primercompletely, the frame extending beyond the edge of the cladder plate atthe initiator side by l inch.

The blasting caps are actuated simultaneously, initiating the primerswhich, in turn, initiate the granular explosive adjacent thereto as thedetonation passes along the periphery of the primers. Initiation of thegranular explosive coupled with the impulse from the primers along theaxis of initiation causes the cladder plates to collide progressivelywith the backer. Examination of the clad product reveals completebonding in the vicinity of the primers, with no evidence of spalling orindentation of the cladder plates. Framing camera measurements made onthe same cladder plate and with the same primer show that a steady-statecollision angle of 6.5 is set up within 0.25 inch from a point on theaxis of the primer, and that a collision velocity of 2100-2400 metersper second is produced fromthe instant of impact.

EXAMPLE 2 2.

The procedure described in Example 1 is followed to produce atitanium-steel-titanium triple clad. The 35-A titanium cladder platesare ll.5 X 12 inches and 0.25 inch thick. The C1008 steel backer plateis 13.5 X 14 inches and 1 inch thick. The standoff is 0.25 inch, and thedetonation velocity 2200 meters per second. The primer described inExample 1 is used on one cladder plate, the smaller base of the conehaving a diameter of 0.625 inch, and the diameter of the cylinder being1.25 inches. The primer used on the other cladder plate is one havingthe same core and sheath as that described in Example l, but one whoseconfiguration is entirely cylindrical (l-inch diameter), the sheathextending the full length of the cylinder. After detonation andcladding, the clad product is inspected and found to-be completelybonded on the side initiated in accordance with this invention. Underthe completely cylindrical initiator, there is a poorly bonded area of15 square inches. g

2. An explosive charge according to claim 1, wherein said cylindricalportion has a diameter about 2- 4 times that of the truncated conicalend of said charge.